This invention relates to apparatus for controlling the time interval between consecutive output pulses of an implanted cardiac pacemaker, and thereby allowing non-invasive electrophysiologic studies, or therapeutic, cardiac stimulation for tachyarrhythmias.
The identification and diagnosis of cardiac tachyarrhythmias and the means of testing the efficacy of antiarrhythmia drug regimens are very often accomplished clinically through the use of electrophysiologic testing. This testing technique, which necessitates the introduction of single or multiple premature stimuli, or short bursts of rapid stimuli has, in the past, been primarily restricted to an invasive approach requiring catheterization.
An earlier attempt in U.S. Pat. No. 4,307,725, issued Dec. 29, 1981 to George E. Sowton et al discloses noninvasive electrophysiologic testing which uses an implanted pacemaker that required an external stimulator to produce short interval pulses on surface skin electrodes. These pulses were detected by the implanted device and used to trigger an output from the implanted device. This technique requires the implanted device to have a short refractory period (typically 180 ms or shorter). Using a short pacemaker refractory period, a normal pacemaker sensing circuit could inadvertently sense "T" or "R" waves, or electrode polarization, and thereby upset the desired stimulation sequence. In addition, when using this technique, the maximum delay between any two pulses is the escape interval of the pacemaker.
The Keller, et al U.S. Pat. No. 4,203,447 describes a technique in which data which has already been transmitted is transferred into permanent memory at the time of an output pulse.
The technique of the present invention allows for changing the internal program on successive pulses. For instance, if two rate programs are transmitted immediately after two successive output pulses, the results would be definite: The Spectrax SXT.TM. pulse generator manufactured by Medtronic, Inc., is an example of a buffered device which may be programmed in accordance with the present invention.
But, if the program transmissions are not synchronized with the output pulses, unexpected results could occur.